Most centrifugal blowers possess a spiral housing which serves to guide the air emerging from the impeller in a substantially tangential direction, to one common discharge opening and to partly convert the velocity head to pressure head by gradually and smoothly reducing the air velocity on its way along the spiral. In high-velocity and high-pressure blowers this process is aided by means of a plurality of preferably curved guide vanes placed around the impeller in the way of the air emerging therefrom, in such a manner as to deflect it and to reduce its velocity. The use of guide vanes results in a higher overall efficiency and smaller dimensions of the spiral casing. Guide vanes are generally not used with smaller blowers, since the small saving in energy does not warrant their additional cost.
On the other hand, the heating of air in domestic appliances, such as room heaters, hair dryers, laundry dryers and others, is generally done by electric resisters placed in the air stream issuing from a centrifugal blower.
In order to save space and to increase heat transfer by turbulance it has been proposed to place electric resistors in the shape of coils or ribbons into the blower casing proper, either along the scroll or as annular elements surrounding the impeller or rotor. Obviously, these resistors disturb and obstruct the otherwise smooth air-flow, while consuming additional energy, and in order to counteract this effect and to reduce the air velocity the scroll dimensions are frequently enlarged; however, this results in a detrimental increase of the overall dimensions of the appliance.
In view of these drawbacks it is the object of the present invention to provide heating elements in a shape suitable for preventing disturbance of the air flow and, in addition, to arrange these in the shape of flow-guide vanes around the circumference of the impeller, with a view to improve the flow and the overall efficiency of the appliance.